This invention relates to electromagnetic devices having encapsulated electrical conductors which are at least partially surrounded by a magnetic flux guide. In particular, this invention relates to electromagnetic devices which are used in high temperature environments.
There are many applications where it is desirable to have electromagnetic devices which can operate in harsh environments. For example, high temperature environments or environments which subject a high degree of vibration on a device. Such applications might include motors, generators, solenoids, valve actuators, pumps and control rod mechanisms etc in aero-engines or nuclear power plants.
Electromagnetic devices having a ferromagnetic flux guide and an electrical conductor insulated by a polymer are generally well known. However, high temperature applications require alternative electrical insulators to replace conventional polymeric materials to prevent electrical and mechanical breakdown at elevated temperatures. Possible replacement electrical insulators are ceramic materials.
Problems can arise with the use of ceramic insulators, and similar alternatives, due to a mismatch in the relative coefficients of thermal expansion of the ceramic and the material which forms the magnetic flux guide. The resulting mismatch in thermal expansion can lead to mechanical and electrical breakdown of the ceramic insulators. These problems are particularly significant in large machines where the differential thermal expansion is increased due to the general increase in the size of the constituent components. Coils produced with ceramic insulation and encapsulants also have significantly lower mechanical compliance than polymer based coils.
Ceramic insulators can also mechanically and electrically degrade when exposed to high levels of vibration over long periods of time, which can limit the applications such insulators can be employed in.